This invention relates to a monolithic semiconductor laser arraya more particularly, to an independently addressable, high density, laser array using a native oxide layer for optical confinement and electrical isolation.
Monolithic arrays of solid state semiconductor lasers are very desirable light sources for high speed laser printing, optical fiber communications and other applications. A common laser structure is a so-called "edge emitting laser" where light is emitted from the edge of a monolithic structure of semiconductor layers.
A ridge waveguide can form the lateral optical confinement structure for the semiconductor laser and thus the ridge precisely defines the origin of the edge emission of light from the semiconductor laser. A ridge waveguide is typically a small ridge of semiconductor material with a generally flat upper surface and sloped sidewalls on top of the active semiconductor layer in a semiconductor laser structure.
Index guiding refers to the use of variations in the optical index of refraction between semiconductor layers of different materials to provide for a waveguide to optically confine the emission of light from one of the semiconductor layers.
Generally, formation of native oxides in a laser is an important step to achieving good electrical and optical confinement in the structure. One approach in oxide formation is commonly known as the "surface-oxidation" technique. Examples of such an approach are described in U.S. Pat. No. 5,327,448 entitled "Semiconductor Devices and Techniques For Controlled Optical Confinement" and U.S. Pat. No. 5,262,360 entitled "AlGaAs Native Oxide," both of which were invented by Holonyak et al.
As discussed in these patents, under the "surface-oxidation" approach, a cap GaAs layer is placed above a thick AlGaAs layer with a high aluminum content, which is deposited above the active layer of a laser structure. Under this "surface-oxidation" approach, the surface of the sample is first patterned with silicon nitride, protecting and exposing parts of the GaAs cap layer. The exposed GaAs areas are then removed by chemical etching exposing the surface of the underlying AlGaAs layer which has a high aluminum content. The sample is then oxidized in water vapor where the oxidation in the AlGaAs layer proceeds downwards from the surface until it reaches the active layer which has a lower aluminum content. Since the active layer has a lower aluminum content, the oxidation process essentially stops when it reaches the active layer, providing electrical and optical confinement to the laser structure.
Another approach towards forming oxides is a so-called "buried-layer" oxidation approach which is described in "Lasing Characteristics of High-Performance Narrow Stripe InGaAs-GaAs Quantum-Well Lasers Confined by AlAs Native Oxide," IEEE Photonics Technology Letters, Vol. 8, No. 2, p. 176 (February 1996) by Cheng et al. Under this approach, an AlAs layer is placed above and below the active layer of a laser structure. Then, grooves are etched, forming an exposed stripe mesa structure between the grooves. As a result of the etching, the AlAs layers sandwiching the active layer are exposed along the sidewalls of the mesa. During an oxidation process, these AlAs layers are oxidized laterally from the sidewalls of the mesa inwards towards the center of the mesa. However, other layers in the structure remain essentially unoxidized since their aluminum content is lower. The oxidized AlAs layers reduce the effective refractive index of the regions both above and underneath them, providing lateral electrical and optical confinement to the sandwiched active layer. Another discussion regarding the "buried-layer" technique is described in "High-Performance Planar Native-Oxide Buried-Mesa Index-Guided AlGaAs-GaAs Quantum Well Heterostructure Lasers," Appl. Phys. Lett. vol. 61 (3), p. 321 (July 1992) by Smith et al.
The key disadvantage of the "buried-layer" approach is the difficulty in controlling the amount of oxidation. Because the oxidation rate of AlAs or AlGaAs with a high aluminum content depends upon aluminum composition and process variations, any variation in aluminum composition or process parameters will be reflected by changes in the oxidation rate, which in turn creates uncertainty in the amount of oxidation. The process is relatively temperature-sensitive. Therefore, when such a technique is applied to forming lasers, the devices typically have manufacturability and yield problems.
With laser arrays, it is desirable to position the laser elements as densely as possible. However, closely spaced elements are difficult to electrically connect and to cool through heatsinking. Furthermore, closely spaced laser elements tend to interact electrically, optically and/or thermally. These interactions, called "crosstalk", are usually undesirable.
Individually, lasers are low power output devices. Arrays of lasers can be used to increase the power output and to simplify optical system design. To provide and maintain good optical alignment of the laser elements of the array with one another and to minimize the assembly involved, arrays have been fabricated so that the laser elements are in a single monolithic semiconductor structure.
Another problem is making each individual laser element in the array independently addressable. As the laser elements are spaced closer together in higher densities, it is progressively more difficult to separately, individually and independently cause each element to emit light.
Accordingly, there is a need for developing a monolithic, independently addressable laser array with accurately defined and controlled oxide regions which provide electrical and optical confinement.